NATURAL ENEMIES OF STINK BUGS IN GEORGIA

Summary

Insecticide use in Georgia cotton has declined dramatically since the late 1980s, with the successful elimination of the cotton boll weevil in the 1990s and the widespread deployment of Bt-transgenic cotton, beginning in 1996. In the late 1980s, growers sprayed insecticides an average of 17 to 20 times per season. By the late 1990s, growers sprayed an average of two to three times per season, making cotton one of the more environmentally-friendly crops in the state. However, in this reduced insecticide environment a complex of stink bug species has emerged as a significant pest threat to Georgia cotton, and other crops. Emergence of these pests threatens to overturn at least some of the insecticide reductions experienced in recent years. The purpose of this project was to characterize the natural enemies of stink bugs in Georgia in an effort to understand how much mortality is inflicted by natural enemies on the stink bugs, and to determine if there might be opportunities to improve the efficiency of important enemies or possibly to import effective enemies from elsewhere to effect greater biological control. Stink bug eggs were parasitized at low to moderate levels in the field by parasitic wasps. In addition, late nymphal stages of the bugs also are parasitized at very low levels by parasitic flies. However, we have recorded a wasp attacking southern green stink bug nyphs for the first time in the Western Hemisphere, and this may offer opportunities to expand biological control of stink bugs. Adult stink bugs are parasitized at low levels early in the season, and at increasingly higher levels throughout the season by parasitic flies. The natural enemy complex currently present in Georgia is very limited in diversity and in efficacy against stink bugs. Fire ants and Conocephalus grasshoppers were found to be significant predators of stink bug eggs, especially ijn peanuts and soybeans. Therefore, in areas where fire ants are abundant and active there may be potential for increased biological control of stink bug populations. However, fire ants were not effective predators in soybeans, and were less effective in cotton than in peanuts. We are cooperating with workers in Europe to evaluate a European parasitic wasp from the Mediterranean region, which attacks immature stages of our principal stink bug pest, the southern green stink bug, for supplementing our native parasite populations.

Situation

The successful eradication of the cotton boll weevil in the 1990s, followed by the widespread deployment of Bt-transgenic cotton in 1996 and subsequently, created an environment in which cotton was transformed from one of the most insecticide-intensive cropping systems in the United States to one that required among the least insecticide usage in the southeastern United States. However, in the 1990s stink bugs became increasingly serious pests of cotton and growers began to apply more insecticides to manage these pests. This increased insecticide use threatened to offset at least some of the benefits that have been derived from boll weevil eradication and the use of Bt-transgenic cotton. Solving the stink bug problem has been compounded by difficulties associated with sampling stink bugs in the field and defining clear treatment thresholds at which insecticide applications should be made. Further, stink bugs range across the landscape, feeding and developing on a variety of cultivated and wild plants. Adults are highly mobile, allowing them to move between fields and between crops and wild systems. Finally, the stink bug problem is compounded by the presence of a complex of stink bug species rather than a single species. The susceptibility of these species to insecticides varies significantly with species, presenting management difficulties to growers.

Response

We continued surveys in 2008 that we had initiated in 2007 to evaluate the complex of natural enemies that attacks stink bugs in Georgia. Our studies concentrated on soybean and cotton fields in Tift, Sumter, and Decatur counties. Stink bugs of all life stages were collected as they were available at each of these locations and the bugs were returned to the laboratory where they were held and assessed for parasitism. In addition, egg masses were placed in cotton fields in Tift County, Georgia, to assess predation of eggs by feral predators. Parasitism of stink bug nymphs and adults was moderate early in the season, but declined in the middle portions of the season. Parasitism rates increased toward the end of the season. Parasitism rates never exceeded 25% at any location during the season, even when parasitism was relatively heavy. Nearly all parasitism of nymphs and adults was attributable to a single fly species, Trichopoda pennipes. We also obtained 15 stink bug nymphs parasitized by a braconid wasp in the genus Aridelus. This is a first record for this parasitism in the Western Hemisphere, and the wasp may be a new species. We have obtained specimens of this wasp for two consectutive years in two different counties, suggesting that it is established in diffuse locations in Georgia. In addition to nymphs and adults, some egg masses also were collected at all locations and held for parasitism. Egg parasitism also was relatively low accounting for less than 10% of egg masses, and all parasitism was due to scelionid wasps. Predation of sentinel egg masses in the field was low in cotton where fire ant populations were suppressed (less than 1%), but loss of southern green stink bug eggs was higher (approximately 20%) where fire ants were present. In contrast, egg predation in peanuts was high (approximately 60%), and inflicted almost entirely by fire ants. Overall, fire ants may play a very significant role in reducing stink bug populations in crop systems in Georgia, particularly in cotton. Stink bug egg predation in soybeans also was quite high (approximately 70%), but there was little evidence for fire ant activity. Instead, the dominant egg predators in soybeans were grasshoppers, especially of the genus Conocephalus. Because of the limited rates of parasitism we observed for nymphal and adult stink bugs, we are pursuing cooperative work with USDA-ARS scientists in Europe (at the European Parasite Laboratory, Montpelier, France) to evaluate a parasitic wasp of Mediterranean origin for possible introduction into the southern United States. Surveys for natural enemies of stink bugs in Georgia will continue for at least one more year to evaluate the diversity and efficacy of native species before we more actively pursue importation of natural enemies from abroad. In addition, when we more fully characterize the natural enemy complex we can begin to examine the ecology of key natural enemies and establish possible mechanisms for fostering populations of those enemies that are particularly promising (such as provisioning of food resources through floral plantings).

Impact

Reliance on natural enemies to manage stink bug populations reduces the need for grower inputs, and can help us maintain the remarkable environmental and economic benefits that reduced insecticide inputs have accrued to Georgia cotton producers and the state of Georgia in recent years. Further, natural enemies work across the landscape so that greater efficacy of the natural enemy complex against stink bugs would benefit a range of crops in addition to cotton with no additional grower inputs. The role fire ants and Conocephalus grasshoppers played as predators of stink bug eggs would indicate that activities that promote these predators in cropping systems also may contribute to reduced stink bug problems. This issue will be evaluated in more detail in the future, but the growing acreage in conservation tillage in Georgia may benefit stink bug management because conservation tillage promotes fire at populations. Finally, the discovery of a new parasitoid that attacks stink bug nymphs may have considerable potential to help suppress stink bug populations. Previously, there are few records of stink bug nymphs being attacked by parasitoids, but we may be able to enhance the activity of this novel parasitoid to reduce stink bug populations on a regional scale

State Issue

Agricultural Profitability and Sustainability

Details

• Year: 2008
• Geographic Scope: Multi-State/Regional
• County: Tift
• Program Areas:
  • Agriculture & Natural Resources